This invention relates to suspending materials and more particularly to suspending materials which are used in fluid suspensions to prevent or substantially reduce the agglomeration of the particles in suspension. This agglomeration phenomenon is particularly marked when fluid suspensions are used in a light valve under the influence of an electric field.
In colloidal suspensions and especially liquid colloidal suspensions the particles in suspension have a tendency to group together to form large clusters of particles. This phenomenon, agglomeration, destroys the uniform distribution of the particles in suspension and in many cases renders the suspension of little value. The problem is particularly pronounced in suspensions that are used in light valves. A light valve is described in more detail in U.S. Pat. No. 3,708,219, assigned to the assignee of the present invention. It usually consists of two transparent flat substantially parallel walls which are separated by a relatively small distance, generally on the order of 0.5 mil to 50 mils, and is sealed around its periphery to form an enclosed cell. Thin, conductive, transparent coatings are applied on the interior surfaces of the walls and the cell is filled with a fluid suspension which may include light polarizing particles. The suspended particles are normally randomly dispersed in the suspension and in this random condition, due to Brownian motion the fluid suspension appears dark, because the particles absorb light and tend to extinguish visible light rays attempting to pass through the suspension. However, when a voltage is applied to the conductive coatings of the light valve (i.e., across the suspension) the particles, which are preferably rod-like, acicular, or otherwise anisometric, align perpendicular to the walls of the cell. In this condition, light passes through the suspension and the suspension appears transparent. It is principally this application of the voltage to the suspension that causes agglomeration of the particles.
There has been substantial research in an effort to develop a light valve suspension where the particles in suspension would stay uniformly distributed and not agglomerate or group together when an electric field is applied. Conventional suspensions will hold particles in suspension for various periods of time with no applied electric field. Examples of such suspensions are given in U.S. Pat. Nos. 1,951,664 Land; 1,955,923 Land; 1,963,493 Land; 3,512,876 Marks; and 3,773,684 Marks. However, when an electric field is applied, the suspending materials in the art, prior to this invention, are unable to maintain the particles in suspension in a dispersed condition; and the particles group together to form agglomerates. Various methods have been developed over the years to attempt to overcome this problem. One such method is described in U.S. Pat. No. 2,481,621 Rosenthal; wherein supersonic waves are transmitted into a liquid suspension to agitate and disorient the suspended particles in order to break up and prevent agglomeration.
Another method is described in U.S. Pat. No. 3,655,267 whereby a high frequency alternating current voltage is used to prevent agglomeration. Other patents propose to use a pulsating voltage as a means of reducing agglomeration. Still others use a smooth generally laminar flow to cause the constant movement of the particles and thereby prevent the particles from agglomerating.
These prior art patents have required the use of electrical or sonic methods or fluid movement to either prevent or break up agglomerates. They each require the use of special equipment in conjunction with the light valve to accomplish the anti-agglomeration function.
Thus, a method of maintaining the dispersed condition of the particles by using a suspending material which maintains such dispersion upon the application of a voltage across the suspension would be extremely useful and of great value in the operation of light valves.